Contemporary 4G telecommunications systems, such as LTE system (Long Term Evolution), have to meet ever increasing needs for higher data rates and improved network capacity. Particularly for environments where the users are highly clustered, a densified infrastructure may be appropriate, where multiple low-power base stations are installed to complement a macro base station which provides basic coverage in the cell. Hence, a heterogeneous telecommunications system is provided, having two—or even more—cell layers. Depending on its transmission power capacity compared to the macro cell, a low-power base station in a heterogeneous telecommunications system is often referred to as a micro base station, pico base station, femto base station, etc. For the rest of the present disclosure, however, the term low-power base station will be used to collectively refer to any such base station.
FIGS. 2A and 2B schematically illustrate an exemplary heterogeneous telecommunications system 200, in which one macro base station 210 and one low-power base station 220 are arranged. Of course, a real-world implantation will contain a large number of macro base stations and a multitude of low-power base stations within at least some of the cells defined by the respective macro base stations. In the illustrated example, the heterogeneous telecommunications system 200 is LTE compliant; hence the macro base station 210 and the low-power base station 220 may alternatively be referred to as evolved Node B:s (eNB:s). The macro base station 210 has a coverage denoted as 202, whereas the low-power base station 220 has a nominal coverage 222, or uptake area, which is essentially confined within the coverage 202 of the macro base station 210.
The basic handover principle in any cellular telecommunications system is that a mobile terminal, from now on referred to as a user equipment or UE, connects to the base station from which the downlink signal strength is the highest. In FIG. 2A, the downlink signal strength is denoted with solid lines. At a first virtual border 286, the downlink signal strength as perceived by a UE 230 is essentially equally strong from the macro base station 210 and the low-power base station 220. Hence, as seen in FIG. 2B, when the UE is at a position 230b, the UE will connect to the macro base station 210, as seen at 232b, since the downlink signal strength from the macro base station 210 is the stronger. Conversely, when the UE is at a position 230a on the other side of the border 286, the UE will connect to the low-power base station 220, as seen at 232a. 
However, due to the difference in transmission power between the low-power base station 220 and the overlying macro base station 210, this basic handover principle may not necessarily result in the UE connecting to the base station to which it has the lowest path loss. In FIG. 2A, the path loss is denoted with dashed lines. At a second virtual border 288, the path loss for the UE 230 is the essentially the same to the macro base station 210 as to the low-power base station 220. In effect, therefore, there is a transition zone 284, outside of the nominal coverage 222 of the low-power base station 220, in which the UE 230 experiences a higher downlink signal strength from the macro base station 210 but a lower path loss to the low-power base station 220.
The coverage or uptake area of the low-power base station 220 can therefore be extended to a broader coverage 224 by including also the transition zone 284. This feature is referred to as Range Expansion (RE) and is available already in the first release of LTE, Rel-8. Known synonyms are Range Extension (RE) and Cell Selection Offset (CSO); the feature will however be collectively referred to as range expansion throughout this document.
Range expansion does not increase the transmission power of the low-power base station 220; instead an offset is added to the received downlink signal strength in the cell-selection mechanism for the UE 230. In the exemplary situation shown in FIG. 2B where the UE 230 is within the transition zone 284, the cell-selection mechanism will cause the UE 230 to select and connect 232′ to the low-power base station 220 by applying range expansion. If range expansion had not been available, the UE 230 would instead select and connect 232 to the macro base station 210. Range expansion is a feature which can be used for the purpose of achieving high uplink data rates, for load balancing between the cell layers (for instance to offload the macro layer), and for improving the robustness by enlarging the low-power base station's coverage to reduce its sensitivity to ideal placement in a traffic hotspot.
Despite the apparent advantages as presented above, the present inventors have identified some drawbacks with range expansion.
In a densely planned network, the uplink to the macro base station can be very good around the low-power base station, and the UE may reach maximum MCS (Modulation and Coding Scheme) and uplink bit-rate to the macro base station. In such a situation there will be no gain from applying range expansion, only a loss in downlink bit-rate. This is so because connecting to the low power base station will degrade the downlink, where, in contrast to uplink, there are always Reference Signals (RS) and control signals transmitted. This results in lower received signal strength and downlink bit-rate. The problem is particularly pronounced for lower frequency bands, e.g. at 800 MHz, with less propagation loss.
Also in more sparsely planned networks, some UE:s may still have a very good uplink to the macro base station in the transition zone. This may for instance be the case for outdoor UE:s with line-of-sight to the macro base station.
There is thus a need for an improved manner in which range expansion is applied in a heterogeneous telecommunications system.